This application claims the priority of Application No. H10-116935, filed Apr. 27, 1998 in Japan, the subject matter of which is incorporated herein by reference.
The present invention relates to an optical transmitter using an optical modulator, and more particularly to, an optical transmitter and optical communication system in which the waveform of output light supplied from the optical transmitter is sufficiently optimized.
In recent years, a network for a basic trunk system is required to transmit signals at a high transmission rate of more than 10 Gbits/s, as a large amount of data, such as image data, is increasingly transmitted. In order to realize such high rate transmission, it is required to modulate input light (carrier) at a high speed. Many methods for realizing high rate transmission have been proposed.
According to a conventional method, a semiconductor laser is directly modulated. However, when the semiconductor laser is modulated at a rate more than 10 Gbit/s, chirping (waveform deformation) occurs in transmission lines. In other words, transmitted light is deformed due to wavelength dispersion of the optical fibers.
A conventional optical transmitter includes a Mach-Zehnder type of optical modulator using LiNbO3 crystal as a light source, hereinafter called LN modulator. Such an optical transmitter is described, for example, in Japanese Patent Laying Open H4-192729 and Technical Report xe2x80x9cOptical Transmitting/Receiving Modulexe2x80x9d FUJITSU 48.5, pp451-456 (09, 1997). According to this kind of optical transmitter, the modulator is required to be driven with driving voltage (modulating voltage) having high amplitude of 4 Vpp to 5 Vpp. In addition, a control circuit, so-called DC drift circuit, is required to compensate the fluctuation of input/output characteristics of the transmitter. Therefore, it is difficult to make the optical transmitter small in size.
For resolving the above mentioned problems, EA (Electro-Absorption) optical modulators have been used. EA optical modulators absorb input light in response to an applied voltage. Such an EA optical modulator is, for example, described in Report of Electro Information Communication Society, LQE95-17 (1995-06) by Yamada et al. and Report of Electro Information Communication Society, 1997 meeting B-10-195, by Takashima et al. In those reports, an optical modulator is integrated with a laser diode of distribution feedback type (DFB-LD) so as to provide a small size of transmitter and to have high power output.
A modulator in which Chirp parameter (coefficient) xcex1 is controlled to be a desirable value is described in Japanese Laying Open H9-179097. According to an optical transmission system described in Japanese Patent Laying Open H9-179079, transmission characteristics are monitored and chirp parameter xcex1 of an EA optical modulator is controlled in response to the monitored values. According to this system, the chirp parameter xcex1 is calculated based on the amount of frequency fluctuation that occurs when light is modulated with a low level signal. In other words, the chirp parameter xcex1 is merely an approximation, which is a static value. Therefore, it is difficult to compensate waveform deterioration sufficiently when the system is actually used, especially when optical fibers having a wavelength dispersion characteristic are employed.
Accordingly, an object of the present invention is to provide an optical transmitter which outputs transmission light having the optimum amount of optical frequency fluctuation.
Another object of the present invention is to provide an optical communication system in which transmission light has the optimum amount of optical frequency fluctuation.
Additional objects, advantages and novel features of the present invention will be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
According to the present invention, the amount of optical frequency fluctuation of a transmission signal is dynamically monitored. The amount of frequency fluctuation, which corresponds to the chirp parameter, is optimized in response to the monitored value. As a result, waveform deterioration of transmission signals is sufficiently compensated.
According to a first aspect of the present invention, an optical transmitter includes a light source emitting input light; and a modulator modulating the input light in accordance with an input electric signal to generate an optical transmission signal. The transmitter also includes a monitor which detects the amount of frequency fluctuation of the optical transmission signal. The modulator is controlled so as to optimize the amount of frequency fluctuation of the optical transmission signal.
According to a second aspect of the present invention, an optical transmitter includes a light source which emits input light for transmission; and an EA (Electro-Absorption) type of optical modulator which absorbs the input light in response to an applied voltage to power-modulate the input light, so that an optical transmission signal is generated. The optical transmitter further includes a bias circuit which applies a bias voltage to the optical modulator; a driver circuit which generates a driving signal (modulating signal) to drive the optical modulator; a control circuit which controls the output power of the light source and the amplitude of a waveform of the driving signal; and an optical frequency fluctuation monitor which detects the amount of frequency fluctuation of the optical transmission signal. The optical modulator is controlled so as to optimize the amount of frequency fluctuation of the optical transmission signal.
According to a third aspect of the invention, an optical transmitter includes a light source which emits input light for transmission; and a Mach-Zehnder type of optical modulator which power-modulates the input light to generate an optical transmission signal, and is provided with first and second electrodes. The optical transmitter further includes a first driver circuit which supplies a modulation signal to the first electrode of the optical modulator; a second driver circuit which supplies a modulation signal to the first electrode of the optical modulator; a bias circuit which applies a bias voltage to each of the first and second electrodes of the optical modulator.
The optical transmitter further includes a control circuit which controls the output power of the light source and the driving amplitude of output waveform of each of the first and second driver circuits; and an optical frequency fluctuation monitor which detects the amount of frequency fluctuation of the optical transmission signal. The control circuit controls at least one of output amplitudes of the first and second driver circuits and levels of the bias voltages applied to the first and second electrodes of the optical modulator.
According to another aspect of the invention, any one of the above described optical transmitters is used in an optical transmission system.